I'm probably going to be implementing aircraft carriers and missile frigates soon. (They'll basically work as air bases tied to sea units. The air base can only be filled up when adjacent to a sea and air base and can only develop one type of air unit, depending on the type of sea unit.)
The current transport unit resembles a carrier and I needed to differentiate the missile frigate from the warship, so here's what the new units will look like once implemented:

From left to right, it's a transport, a warship, a carrier, and a frigate.